The present invention relates to a diagnostic subroutine for testing an oxygen sensor heater of an internal combustion engine.
Internal combustion engines can include an exhaust oxygen sensor, referred to as an O2 sensor, which can be located in the engine exhaust manifold or exhaust piping to monitor the percentage of oxygen contained within the exhaust gases. The oxygen sensor provides a feedback signal in a closed-loop engine control system where an air-fuel mixture ratio is maintained as close to stoichiometric as possible using the signal from the oxygen sensor. Electronic control modules or units are in wide use with two types of oxygen sensors; namely, an unheated type of O2 sensor; and a heated type of O2 sensor. The unheated type of oxygen sensor does not output a voltage signal to the onboard electronic control unit until the sensing unit has reached operating temperature. The heated type oxygen sensor is fully operational within a few seconds of engine startup, regardless of the exhaust gas temperatures. When the sensing element has not yet reached an operating temperature with the heater energized or activated at the initial engine startup, the sensing element is in an inactive state. When the sensing element is in the inactive state, no diagnostic system can make an accurate diagnosis of deterioration of the oxygen sensor.
Oxygen sensors monitor the by-products of combustion in order to regulate the air-fuel mixture. Proper regulation of the air-fuel mixture is necessary to achieve clean burning of the fuel. Strict emission standards are difficult to meet if the fuel is not cleanly burned.
The oxygen sensor generates an output voltage depending on the content of oxygen and the fuel-air mixture at the exhaust. If the exhaust gas is rich in oxygen, the sensor will produce a low voltage, close to zero volts. If the exhaust gas is rich in fuel, the sensor will produce a voltage close to one volt. The output voltage and internal resistance of an oxygen sensor varies with the temperature and the age of the sensor. A cold oxygen sensor has a very high internal resistance, which decreases significantly once the sensor reaches the operating temperature of approximately 300xc2x0 Celsius. A cold oxygen sensor is unreliable during the engine warm-up phase. During this period, the engine operates in an open loop where data from the oxygen sensor is not used to regulate the air-fuel mixture. The addition of a heater to the oxygen sensor allows the sensor to reach an operating temperature faster than the sensor would if heated only by the engine exhaust gases. The heater allows the internal combustion engine to reach a closed loop mode of operation where the signal from the oxygen sensor is used to regulate the air-fuel mixture more rapidly than relying on engine exhaust gases alone to heat the oxygen sensor. Oxygen sensors become less reliable with age, because physical wear and chemical contamination effect the output voltage and internal resistence of the sensor. Failed oxygen sensors cause an internal combustion engine to run inefficiently, and have an adverse impact on the performance of the vehicle. The amount of air pollutants produced by a vehicle increases directly as a result of an unclean burn. A failed sensor can also increase fuel consumption.
When the heater fails, an oxygen sensor must be heated sufficiently by the engine exhaust until the oxygen sensor reaches the optimal operating temperature. Existing on-board diagnostic routines require that the vehicle be xe2x80x9ccold soakedxe2x80x9d before the diagnostic routine will be executed. The on-board oxygen sensor heater time-to-activity diagnostic currently requires the vehicle to be cold soaked for approximately eight hours before diagnosing the heater. Vehicle owners may consider it inconvenient when required to leave a vehicle for an extended period of time in order to perform the known oxygen sensor heater diagnostic.
The oxygen sensor heater service bay test according to the present invention is used to diagnose an oxygen sensor heater in a shorter period of time than required for known diagnostic tests. Currently the existing on-board diagnostic requires an eight hour cold soak before the test is initiated. The present diagnostic routine generally requires that the vehicle be running in park or neutral in the service bay for a calibrateable or predetermined period of time to insure that the oxygen sensors are sufficiently warm. The vehicle is then turned off, and the key is returned to the key-on/engine-off position. The service tool is connected to the internal combustion engine and sends a message to the programable computer module to initiate the heater service bay test. The programable computer module will acknowledge the message and command the test to continue only if the oxygen-sensor-heater-inspection-maintenance-ready flag indicates that the test has never been run before. Once the test has been commanded to run, the service bay test will disable the fuel injectors so that no fuel is delivered when the engine is cranked. The operator is then instructed to crank the vehicle engine to insure that the oxygen sensors will be exposed to a lean environment. The service bay test then measures the sensor voltage. If the voltage is below a calibration or predetermined threshold value, the test will be armed. If the voltage is not below the calibration or predetermined threshold value, the test will be aborted and the operator is instructed to repeat the procedure from the point where the service tool is connected. After the test is armed, the service bay test monitors the sensor voltage. If the sensor voltage reaches a calibratable or predetermined threshold voltage level after a calibratable or predetermined time duration, the sensor would be considered as failing the diagnostic routine. If the sensor voltage does not reach the calibratable voltage level after the calibratable time duration, the sensor is considered as passing the diagnostic routine. At this point, the oxygen-sensor-heater-inspection-maintenance ready flag is set to indicate that a diagnostic test has been performed on this oxygen sensor heater.
The oxygen sensor heater service bay test allows the oxygen sensor heater to be diagnosed within 20 to 30 minutes. The test according to the present invention will improve customer satisfaction by significantly reducing the time required for the customer to have the vehicle at the service center. The test insures that the service center is capable of setting the inspection-maintenance-ready flag for the oxygen sensor heater diagnostic. The test or diagnostic routine, according to the present invention was developed in an effort to reduce the time required to diagnose and service a vehicle with a broken oxygen sensor heater. The test or diagnostic routine according to the present invention can not be used as the on-board diagnostic, since the enabled conditions would not normally be encountered during normal use of the vehicle by the customer.
The present invention allows the oxygen sensor heater to be diagnosed in a timely manner while the vehicle is in for service. The present invention diagnoses the oxygen sensor heater by observing the sensor voltage in a lean environment with the engine off, after the engine has been warmed up. The sensor heater will be considered to have failed the diagnostic routine if the sensor voltage does not remain lean (or low) for a predetermined time.
A diagnostic routine according to the present invention tests an oxygen sensor heater of an internal combustion engine by preheating the oxygen sensor heater, measuring a voltage signal from the oxygen sensor heater after a predetermined period of time, and determining if the voltage signal is greater than a threshold value after the predetermined period of time has elapsed. If the voltage signal is greater than the threshold value, the oxygen sensor heater fails the diagnostic routine. If the voltage signal is not greater than the threshold value, the oxygen sensor heater passes the diagnostic routine.
Other objects, advantages and applications of the present invention will become apparent to those skilled in the art when the following description of the best mode contemplated for practicing the invention is read in conjunction with the accompanying drawings.